This invention relates to a novel automated system for controlling the amount of refrigerant charge in a heat pump refrigerant circuit. More specifically, it is directed to a system which varies the charge during steady state operation to maintain a desired operating characteristic for the heat pump as environmental conditions vary, and which is also adapted to add or withdraw charge from the circuit during changes in the operating state of the heat pump, to minimize adverse consequences of either excess charge or incorrectly located charge during the changes in operating state.
A heat pump for space conditioning conventionally is formed of an interconnected refrigerant circuit comprised of indoor and outdoor heat exchange coils, a refrigerant compressor, a switch-over valve to reverse the flow of refrigerant through the circuit and tubing to provide suitable interconnecting refrigerant flow lines. A first of the flow lines connects one end of each of the heat exchange coils via the compressor and switch-over valve, while the other end of each of the coils is directly interconnected by a second of the flow lines. Conventionally, two separate fluid expansion devices are included in the directly interconnecting flow line, one near each of the heat exchange coils. Typically, these expansion devices are comprised of capillary tubing, expansion valves or a combination of both, depending on the design of the particular heat pump.
It is well known in the art that, for a given heat pump design, optimum operating efficiency is achieved by matching the amount of refrigerant charge to the load encountered by the heat exchange coils. Therefore, as environmental conditions vary, it is desirable to correspondingly add or withdraw charge from the circuit to adapt the system for optimum operation based on the change in load conditions caused by the changed environmental conditions. Moreover, it is desirable that excess refrigerant liquid in the circuit be isolated during the off condition of the heat pump to prevent the charge from migrating to the compressor, and in this way, to minimize the probability of "slugging" during start-up (i.e., injecting incompressible liquid refrigerant into the compressor). Additionally, it is necessary when operating in the heating mode to periodically reverse the operation of the refrigerant circuit to defrost the outdoor coil. It is desirable to anticipate where the refrigerant liquid should be when switching occurs into and out of defrost so that the charge can be suitably located in the system to minimize defrost time and also to minimize start-stop transient losses.
Arrangements have been proposed in the past for controlling the amount of refrigerant charge in a heat pump refrigerant circuit. In general, the prior arrangements have not been satisfactory in providing overall control of refrigerant charge in a circuit in response to changing environmental conditions to achieve optimum coefficient of performance during steady state operation, while also varying refrigerant charge during transient operation to optimize seasonal performance characteristics. One example of such an arrangement is illustrated by U.S. Pat. No. 3,264,838--S. C. Johnson, issued Aug. 9, 1966, in which a charge modulation receiver for a heat pump system is connected, at one end, to the high pressure line intermediate the expansion valves and, on the other end, to the low pressure suction line leading directly to the compressor. Pressure responsive inlet and outlet valves control bleeding of refrigerant out of the refrigerant circuit from the high pressure line and control the supply of refrigerant back into the refrigerant circuit at the compressor suction line. Such an arrangement works solely on the pressure differential between the high and low sides of the refrigerant circuit and is not able to control refrigerant charge for optimum superheating or subcooling as environmental conditions vary. In essence, such an arrangement merely changes the level of refrigerant charge to one of two levels, depending on whether the heat pump system is operating in the heating or cooling cycle. It has the added disadvantage of always returning the refrigerant charge to the suction line, making the arrangement susceptible to compressor slugging unless special precautions are observed.
It is, therefore, an object of the present invention to provide a heat pump refrigerant charge control system which obviates one or more disadvantages of prior known systems.
It is another object of the invention to provide such a charge control system which is capable of maximizing system performance characteristics for different environmental conditions by optimizing the superheating of gas out of the evaporator heat exchanger, and by optimizing the subcooling of liquid out of the condenser heat exchanger.
It is another object of the invention to provide a heat pump refrigerant charge control system which improves heat pump performance and reliability by anticipating changes in the operating function of the heat pump, such as start-up or stopping, or entering/leaving a defrost cycle, to change the location of the refrigerant charge to minimize the adverse effect on performance normally caused by these transient conditions.
It is yet another object of the invention to provide a heat pump refrigerant charge control system in which refrigerant is returned to the refrigerant circuit without increasing the risk of compressor slugging on start-up.